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Photodynamic Therapy (PDT)

Light is not a continous wave but a train of tiny energy packets or light quanta.  These light quanta are known as photons.

When a photon of visible light encounters a molecule, it is possible that the energy of the photon is given to the molecule and the photon is annihilated.  The molecule becomes energized or in an “excited” state. 


As a result of light being absorbed, a chemical bond in a molecule can be broken and a new one can be formed; this is called a chemical reaction. Alternatively, the molecule relaxes back to its unexcited (“ground”) state and emits light.

A molecule can have many excited states; these excited states are intrinsically unstable and anxious to lose their energy and return to the ground state.  Excited state molecules are also much more reactive than ground state molecules.  This is the basis for all types of photochemistry, ranging from photosynthesis and vision to vitamin D synthesis, medical diagnosis and treatment (for example, light treatment of hyperbilirubinemia and photodynamic therapy of cancer).

Photochemist Michael A. J. Rodgers is Bowling Greenís first Eminent ScholarPhotodynamic therapy can be defined as the use of light to induce reactions in the body which help treat diseases in patients.

Photochemist Michael A. J. Rodgers is Bowling Green’s first Eminent Scholar. He studies the nature of rapid dynamic events within molecules that are induced by the absorption of light. He has pioneered the development of instrumentation and methodologies for using fast kinetics to study photochemical processes. His work on new compounds and techniques for use in a variety of cancer therapies (i.e., photodynamic therapy) is highly original and has attracted widespread attention in the scientific community.

Here is how PDT works:

  • A chemical which is not harmful to the body in its original state is administered to the patient
  • Light exposure (often from a laser) of the abnormal tissue containing the chemical activates it, causing it to change into a poison that destroys the irradiated tissue
  • Most desirable chemicals are those that concentrate in tumors (and certain other kinds of proliferating tissue) rather than the surrounding healthy tissue, which remains unaffected
  • Everything happens fast (trillionths of a second)
  • Special laser equipment is required to “see” molecules react so fast


Also see the Erythrocyte diagram in Additional Reading.